1. Field of the Invention
The present invention relates generally to a spread spectrum scheme, and more particularly, to a rake receiver that executes, as a reception diversity scheme in a multi-path environment, maximal-ratio composition in the time domain of signals arriving at an antenna with various differences in delay time thereof caused by multiple reflections of propagation paths of the signals.
2. Description of the Related Art
Spread spectrum or spread spectrum communication scheme is utilized extensively as a basic technique for mobile communication. In the direct spread (DS) scheme as the simplest model of the spread spectrum communication, an information signal is transmitted to the receiving side after the spectrum of the information signal is spread by modulating, that is, multiplying the information signal to be transmitted by a PN signal having the chip width Tc of 1/100 to 1/1000 of the cycle T of the information signal to be transmitted as a spread signal.
On the receiving side, the signal component is detected from the signal buried in noises by inverse spreading. The inverse spreading basically refers to executing demodulation by multiplying a received signal by a same PN signal having the same phase as that of the PN signal in the received signal.
However, in a multi-path environment for many reflected waves to be present in addition to a direct wave, it is necessary to detect a true signal component by composing appropriately signals received with various differences in delay time.
As one of such conventional techniques, a rake scheme can be listed. “Rake” means a rake in English and the rake scheme is a diversity scheme for executing the maximal-ratio composition by collecting power dispersed due to the delay dispersion of transmission paths, into one like a “rake”.
In a conventional rake receiver, a desired signal is demodulated by finding a plurality of path timings at which multiple paths arrive using a known signal, informing a demodulator of these path timings, executing inverse spreading at these timings in the demodulator and composing signals of the multiple paths.
FIG. 1 is a block diagram showing generally an example of the construction of a rake receiver as, for example, a mobile communication terminal. In the figure, the receiver has an antenna 100, a wireless receiving unit 101, an A/D converting unit 102, a searcher 103 for detecting a plurality of timings of the multiple paths and an inverse spreading timing generating and inverse spreading unit 104 for executing an inverse spreading to the plurality of paths according to the timings of the plurality of paths detected by the searcher 103.
The receiver further has a signal composing unit 105 for composing signals of the plurality of paths obtained by the inverse spreading timing generating and inverse spreading unit 104, a signal processing unit 106 such as a channel codec for receiving an output of the signal composing unit 105 and outputting received signals to a display, speaker, etc., and a level measuring unit 107 for measuring the level of the received signals of the plurality of paths, providing reliability degree information and signal level information to the signal composing unit 105 and providing to a transmission unit 108 control information of transmission power to a base station.
The transmission unit 108 transmits input from a keyboard or a microphone, from the antenna 100 through a duplexer 109 in response to the control information from the level measuring unit 107.
FIG. 2 is a block diagram of the detailed construction of the inverse spreading timing generating and inverse spreading unit 104 of FIG. 1, that is, a signal demodulating unit. In the figure, the signal demodulating unit comprises a spread code generator 110, a plurality of delay control units 111-1 to 111-n and a plurality of correlators 112-1 to 112-n corresponding thereto.
The spread code generator 110 generates a code for inverse spreading. The plurality of delay control units 111-1 to 111-n control respectively delay operations of the plurality of correlators 112-1 to 112-n corresponding respectively to timings t1 to 1N of the multiple paths detected by the searcher 103. Each of the correlators 112-1 to 112-n executes inverse spreading on the received signals from the A/D converting unit 102 according to the inverse spread timings controlled by the corresponding delay control units 111-1 to 111-n.
Thereby, the correlators 112-1 to 112-n respectively provide inverse spread signals 1 to N to the signal composing unit 105 and the signal composing unit 105 composes these signals and outputs a demodulated signal.
Such an inverse spread signal includes a channel estimation signal corresponding to a propagation coefficient of each of the multiple paths.
As described above, for example, in FIG. 2, inverse spreading is executed using the timings themselves of each path of the multiple paths. When inverse spreading is executed at a timing, signals corresponding to paths other than the path of this timing are all interference. Especially, in the case where an orthogonal spread code is used for a plurality of channels in a downlink from a base station in the CDMA scheme, a problem exists that the reception property is degraded due to the multi-path interference.
Considering the above point, the inventor has previously proposed a rake receiver capable of suppressing multi-path interference when the spread spectrum scheme is used in a multi-path environment in Japanese Patent No. 2001-332510.
Here, the schematic construction of such a rake receiver as proposed previously will be described. FIG. 3 is a block diagram of the principle construction of a rake receiver constituting a spread spectrum communication system in a multi-path environment, previously proposed.
In FIG. 3, path timing detecting 1 correspond to, for example, the path searcher 103 of FIG. 1 and FIG. 2 detect timings of, for example, N paths.
Inverse spreading timing setting 2 set the detected timings of the paths as timings for inverse spreading, that is, timings for demodulating spread encoding signal by multiplying an inverse spread code. Concurrently, settings are made to all combinations of two (2) paths such that, taking the center at a timing of one (1) path of arbitrary two (2) paths, two (2) timings at positions symmetrical to the timing of the other path on the time axis by the delayed time of the timings of the two (2) paths are timings of the inverse spreading.
A plurality of correlators 3-1 to 3-n respectively obtains an inverse spreading signal of a signal resulted from, for example, A/D conversion of a signal sent from the transmitting side in response to each timing having been set. Signal composing 4 compose outputs of the plurality of correlators 3-1 to 3-n and output a demodulated signal.
As described above, in the present invention proposed in the previous application, interference component contained in a desired signal is reduced using a multi-path interference correlative signal (MICS) reproduced using only information of selected two (2) paths.
However, as described above, in the present invention of the previous application, a drawback is recognized that the effect of the reduction of the interference component becomes smaller as the number of the paths increases because the information of only selected two (2) paths is utilized when the interference component is reproduced. That is, information that must be contained in paths other than the noted two (2) paths can not be utilized.